Selective Triplet Exciton Formation in a Single Molecule

Result of the Month

Scanning Tunnelling Luminescence (STL) optics setup combined with Scienta Omicron LT STM.

STL fluorescence and phosphorescence spectra of the PTCDA on NaCl(3ML) atop Ag(111) system. a, Schematic of STL measurement. b, STL spectrum of a PTCDA molecule (V = −3.5 V, I = 50 pA, exposure time t = 60 s) at low energy resolution; see below for red and blue shaded regions. The tip position is shown as a red dot in the inset STM image (4 nm × 4 nm, V = 1.0 V, I = 5 pA). c, d, STL spectra at medium energy resolution (V = −3.5 V, I = 50 pA, t = 180 s): c, photon energy 2.37–2.52 eV (blue region in b); and d, photon energy 1.25–1.40 eV (red region). e, Peak widths (shown by horizontal arrows) of the 0–0 transitions (I = 50 pA, t = 600 s) of fluorescence (blue, V = −3.5 V) and phosphorescence (red, V = −2.5 V) at high energy resolution. Black lines show Lorentzian fitting results.

Phosphorescence STL map of the PTCDA on NaCl(3ML) atop Ag(111) system. a, Phosphorescence STL spectra were measured at selected pixels in a 3.3 nm × 3.3 nm area (shown) with V = −2.5 V, I = 30 pA, t = 30 s per pixel. The intensities of the 0–0 peak of phosphorescence are measured by Lorentzian fitting and plotted (in counts; see color scale at bottom right). Structure at top right shows molecular direction. b, dI/dV spectra obtained at four selected points (called #1, #2, #3 and #4) in PTCDA; spectra are labelled with point number. Tip positions are shown as colored dots and numbered in the inset STM image (V = −2.5 V, I = 10 pA).

Author: Yousoo Kim Institute: ''Surface and Interface Science Laboratory, RIKEN, Wako, Japan'' URL: https://doi.org/10.1038/s41586-019-1284-2
Date: 1/2020 Instruments: LT STM Lab

The exciton formation in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs). According to a simple model based on spin statistics, the injected charges form singlet (S₁) excitons and triplet (T₁) excitons in a 1:3 ratio¹. After the first report of highly efficient phosphorescence OLED², effective use of T₁ has been the primary strategy for increasing the quantum efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage. Because T₁ excitons have lower energy than S₁ excitons owing to the exchange interaction, use of the energy difference could—in principle—enable exclusive production of T₁ excitons at low OLED operating voltages.

However, a way to achieve such selective and direct formation of these excitons has not yet been established. In this work, we report a single-molecule investigation of electroluminescence using a scanning tunneling microscope (STM)^3-5 and demonstrate a simple way for selective formation of T₁ excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule adsorbed on a three-monolayer (3ML) NaCl film grown on Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T₁ excitons without creating their S₁ counterparts. The sample voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T₁ excitons in this system, which can be rationalized by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that device designing with consideration of the exchange interaction would realize a novel OLED with a lower operating voltage.

Additional Information

Authors

Kensuke Kimura^1,2, Kuniyuki Miwa^1,3,5, Hiroshi Imada^1*, Miyabi Imai-Imada^1,2, Shota Kawahara¹, Jun takeya², Maki Kawai^2,4, Michael Galperin^3* & Yousoo Kim^1*

Institutes

1) Surface and Interface Science Laboratory, RIKEN, Wako, Japan.

2) Department of Advanced Materials Science, Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Japan.

3) Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, USA.

4) Institute for Molecular Science, National Institutes of Natural Sciences, Okazaki, Japan.

5) Present address: Department of Chemistry, Northwestern University, Evanston, IL, USA.

Name and Email of Corresponding Author

Hiroshi Imada, Michael Galperin & Yousoo Kim

himada@riken.jp ; migalperin@ucsd.edu ; ykim@riken.jp

URL of Institute Web-pages

Kim lab.

http://www2.riken.jp/Kimlab/

RIKEN

https://www.riken.jp/

Publication(s)

Kimura, K., Miwa, K., Imada, H. et al. Selective triplet exciton formation in a single molecule. Nature 570, 210–213 (2019)

 doi:10.1038/s41586-019-1284-2

URL of Journal(s)

www.nature.com/  

References

1) Köhler, A. & Bässler, H. Triplet states in organic semiconductors. Mater. Sci. Eng. R Rep. 66, 71–109 (2009).

2) Baldo, M. A. et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 395, 151–154 (1998).

3)  Imada, H. et al. Real-space investigation of energy transfer in heterogeneous molecular dimers. Nature 538, 364–367 (2016).

4) Imada, H. et al. Single-Molecule Investigation of Energy Dynamics in a Coupled Plasmon-Exciton System. Phys. Rev. Lett. 119, 013901 (2017).

5) Miwa, K. et al. Many-Body State Description of Single-Molecule Electroluminescence Driven by a Scanning Tunneling Microscope. Nano Lett. 19, 2803–2811 (2019).